Hair curling appliance power control circuit

ABSTRACT

A power control circuit for supplying electrical power from a power source to an electrical appliance having a heating element, which circuit comprises an input for connection to a power source, an output for connection to a heating element, and supply elements between the input and the output for supplying power at a first, relatively higher level for a predetermined start-up time interval, and at a second, relatively lower level after the predetermined start-up time interval has elapsed. 
     After the start-up time interval, first timing elements brings into operation second timing elements which switches a silicon controlled rectifier off or on, via a NAND gate and transistor to supply power intermittently to the heating element.

FIELD OF THE INVENTION

The present invention relates to a power control circuit for supplying electrical power from a power source to an electrical appliance having a heating element.

SUMMARY OF THE INVENTION

According to the invention there is provided a power control circuit for supplying electrical power from a power source to an electrical appliance having a heating element, the circuit comprising an input for connection to a said power source, an output for connection to a said heating element, and supply means between the input and the output for supplying power at a first, relatively higher level for a predetermined start-up time interval, and at a second, relatively lower level after the predetermined start-up time interval has elapsed.

Preferably, power is supplied continuously to the heating element during the start-up time internal and is supplied intermittently thereafter so as to provide a timeaveraged lower level of power.

More preferably, said supply means comprises first timing means for determining the start-up time interval, and second timing means for providing a signal to control the supply of power to a said heating element.

Preferably, the second timing means is in use inhibited from providing the signal by the first timing means during the start-up time interval.

The invention also provides an electrical hair curler incorporating such a power control circuit.

Other preferred features and advantages of the invention will be apparent from the following description and the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of an embodiment of a power control circuit in accordance with the invention, and

FIG. 2 is a perspective view of an electrical hair curler incorporating the power control circuit of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, FIG. 1 shows an embodiment of a power control circuit 10 in accordance with the invention, for supplying electrical power from a power source V_(ac) to a heating element HT₁ of an electrical hair curler 20 (FIG. 2) which is also in accordance with the invention. The power control circuit 10 comprises an input 11 for connection to the power supply V_(ac), an output 12 for connection to the heating element HT₁, and supply means 13.

The heating element HT₁ is connected in series with a silicon controlled rectifier SCR₁ across the output of a bridge rectifier D₁ -D₄ whose input 11 is connected to the power supply V_(ac).

SCR₁ is switched on and off to control the supply of power to the heating element HT₁. Switching of SCR₁ is controlled by a NAND gate A₄ and transistor Q₁. The inputs P₁ and P₂ to NAND gate A₄ are controlled respectively by a turn-on/turn-off circuit 16 and timing circuits 14 and 15. Circuit 16 serves to control the initial turn on of the power control circuit 10 which is operated manually by a user at switch SW₂ and also turned off at switch SW₁. A master timer 17 in circuit 16 switches off the supply of power to heating element HT₁ automatically after a predetermined time interval set by capacitor C₅ and resistor R₈, e.g. after from 30 to 90 minutes, which can be factory set.

On initial turn on at switch SW₂, the first and second timing circuits 14, 15 are, in effect, bypassed and the output from gate A₄ is set low to turn off the transistor Q₁ and hence turn on SCR₁ to supply power continuously to heating element HT₁. Timing circuit 15 comes into effect after a start-up period determined by timing circuit 14. When timing circuit 15 is brought into effect, it controls gate A₄, switching its output alternately high and low, thus turning transistor Q₁ and hence SCR₁ off and on. Hence the time-averaged power fed to heating element HT₁ is reduced according to the on-off time of SCR₁.

The power control circuit 10 and its operation will now be described in more detail.

First timing means 14 includes a capacitor C₄, a charging path a-b formed by a diode D₁₀ and a resistor R₁₂ for the capacitor C₄, and a discharging path c-d formed by a diode D₉ and a resistor R₁₁ for the capacitor C₄. The second timing means 15 is an oscillator which includes a logic NAND gate A₃, a capacitor C₃, a charging path e-f formed by a diode D₆ and a resistor R₁₀ for the capacitor C₃, and a path g-h formed by a resistor R₉ through which the capacitor C₃ can be charged or discharged.

The NAND gate A₃ has an output P₄ and two inputs P₅ and P₆ to which the capacitors C₄ and C₃ respectively are connected. The two paths e-f and g-h are both connected across the input P₆ and the output P₄ of the NAND gate A₃ so that the second timing means will oscillate, i.e. the capacitor C₃ charging and discharging alternately, when the input P₅ is at logic high.

The output signal of the second timing means 15 is taken at the output P₄ of the NAND gate A₃, and is in turn fed to an input P₂ of a logic NAND gate A₄. An output P₃ of the NAND gate A₄ serves to provide a firing signal which is fed to an inverter formed by a NPN transistor Q₁ and resistors R₃, R₄, R₅ and R₆, connected in the usual manner. The inverted firing signal is taken at circuit node i between the two resistors R₄ and R₅, and is fed to the gate terminal of a silicon-controlled rectifier SCR₁.

The SCR₁ and the heating element HT₁ are connected in series across the anode and cathode of a bridge rectifier D₁ -D₄. The two a.c. inputs of the bridge rectifier D₁ -D₄ serve as the input 11 of the power control circuit 10, and the bridge rectifier D₁ -D₄ converts an a.c. voltage received from the power source V_(ac) into a pulsating d.c. voltage, which is in turn fed by means of a diode D₁₁ and a resistor R₂ to a smoothing capacitor C₁. A zener diode D₅ is connected across the capacitor C₁ for limiting and stabilising the d.c. voltage which is in turn applied to the heating element HT₁ by means of the SCR₁.

Two fuses F₁ and F₂ are connected respectively at the anode and cathode of the rectifier SCR₁ for protection. A neon indicator lamp N₁ is connected in series with a resistor R₁ across the heating element HT₁ for indicating when power is being supplied to the heating element HT₁, i.e. when SCR₁ is conducting.

The supply means 13 further comprises a NAND gate A₂, third timing means 17 and manually operable switch means 18 for controlling the operation thereof. The NAND gate A₂ is connected at its output P₁₀ to circuit nodes a and c for controlling the charging and discharging of the capacitor C₄, and to an input P₁ of the NAND gate A₄ for controlling the provision of the firing signal by the NAND gate A₄.

The third timing means 17 includes a capacitor C₅ and a charging path f-j for the capacitor C₅, the charging path f-j consisting of a diode D₇ and a resistor R₈. Through the path f-j, the capacitor C₅ can be charged whenever the output P₄ of the NAND gate A₃ is at logic high. The capacitor C₅ is connected to inputs P₈ and P₉ of the NAND gate A₂.

A pushbutton switch SW₂ is connected across the capacitor C₅ for quick discharging of the capacitor C₅ as hereinafter described.

The manually operable switch means 18 includes a capacitor C₂, a pushbutton switch SW₁ connected across the capacitor C₂, a resistor R₇ connecting the capacitor C₂ to voltage source V_(DD), a logic NAND gate A₁ connected at its inputs P₁₂ and P₁₃ to the capacitor C₂, and a diode D₈ connecting an output P₁₁ of the NAND gate A₁ to the capacitor C₅.

FIG. 2 shows an electrical hair curler 20 having a handle 21 in which the power control circuit 10 is housed. Otherwise the curler 20 is of conventional construction and comprises a rod-shaped body 22 in which the heating element HT₁ of the circuit 10 is located, and a plate 23 hinged to the body 22, the plate 23 being co-operable with the body 22 to grip hair. The two switches SW₁ and SW₂ and the indicator lamp N₁ are located on the handle 21, and the input 11 of the circuit 10 is connected to a power plug 24 by means of wire leads 25.

In use, the power plug 24 is connected to the power source V_(ac), and immediately the inputs P₁₂ and P₁₃ of the NAND gate A₁ are effectively earthed by the capacitor C₂, which initially is fully discharged. The output P₁₁ of the NAND gate A₁ is therefore initially at logic high, and this initiates quick charging up of the capacitor C₅, causing the output P₁₀ of the NAND gate A₂ to go logic low almost instantaneously. This causes the NAND gate A₄ to provide a logic high output, and which in turn turns on the transistor Q₁ to inhibit the conduction of the SCR₁. As a result, the heating element HT₁ is not energised.

When the power source V_(ac) is connected, the capacitor C₂ starts to be charged through resistor R₇. Resistor R₇ and capacitor C₂ are arranged so that approximately 1 second later capacitor C₂ will be fully charged after connection to V_(ac), causing the output P₁₁ of the NAND gate A₁ to go logic low. Under this condition, the manually operable switch means 18 will have no further effect on the capacitor C₅ until it is manually activated.

To energise the heating element HT₁, the switch SW₂ is momentarily closed to discharge quickly the capacitor C₅, whereupon the output P₁₀ of the NAND gate A₂ goes logic high. The capacitor C₄ starts to be charged via the charging path a-b from substantially ground level. As the input P₅ of the NAND gate A₃ is effectively earthed, by the as yet fully discharged capacitor C₄, the second timing means 15 will be controlled by the first timing means 14 to continue to provide a logic high output at the output P₄ of the NAND gate A₃. At this time, the inputs P₁ and P₂ of the NAND gate A₄ are both at logic high, and the NAND gate A₄ provides a logic low output. This causes the transistor Q₁ to turn off, and which in turn causes the SCR₁ to conduct to apply continuously the rectified d.c. voltage to the heating element HT₁.

This operating condition continues until the capacitor C₄ is sufficiently charged to provide a logic high capacitor voltage after a certain start-up time interval has elapsed, which is determined by the values of the resistor R₁₂ and the capacitor C₄. At this time the capacitor C₄ raises the input P₅ of the NAND gate A₃ to logic high level.

When the output P₄ of the NAND gate A₃ is at logic high, the capacitor C₃ will be charged via the paths e-f and g-h until it is sufficiently charged to raise the logic input P₆ applied to the NAND gate A₃ to logic high. At this time, the output P₄ of the NAND gate A₃ goes logic low. The capacitor C₃ starts to discharge via the discharging path g-h until it discharges sufficiently to lower the logic input P₆ applied to the NAND gate A₃ to logic low, whereupon the capacitor C₃ starts to be charged again as the logic output of the NAND gate A₃ is now at logic high. Therefore, the second timing means 15 oscillates to provide an alternating logic signal at the output P₄ of the NAND gate A₃, and hence at the output P₃ of the NAND gate A₄.

As described above, the SCR₁ will conduct when the output P₃ of the NAND gate A₄ is at logic low, and will not conduct when the output P₃ is at logic high. It follows that the SCR₁ will switch between conducting and non-conducting states when the second timer means 15 oscillates.

It is apparent that the SCR₁ will conduct continuously during the start-up time interval, and in duty cycles thereafter controlled by second timing means 15. The heating element HT₁ will therefore receive full power during the start-up time interval in order to reach its operating temperature as quickly as possible, and will receive reduced power thereafter in order to maintain the operating temperature. In this particular embodiment, the start-up time interval can be set from 30 to 90 seconds by the resistor R₁₂, and the full power and the reduced, operating power ratings are arranged respectively to be 80W and 20W, the latter being provided by the second timing means 15 at an output signal of mark-to-space ratio of 1:3.

Immediately after the switch SW₂ is momentarily closed, the capacitor C₅ is graduately charged via the charging path f-j whenever the output P₄ of the NAND gate A₃ is at logic high. When the capacitor C₅ is eventually charged to provide a logic high capacitor voltage, the output P₁₀ of the NAND gate A₂ will go logic low, and in turn the output P₃ of the NAND gate A₄ will go logic high, inhibiting the conduction of the SCR₁. Therefore, the heating element HT₁ will automatically be switched off after having been energised for a certain period of time. In this particular embodiment, this time period can be set in the range of 60±15 minutes by the resistor R₈.

The heating element HT₁ can also be switched off manually by momentarily closing the switch SW₁. When the switch SW₁ is closed, the capacitor C₂ will be quickly discharged so as to produce a logic high output at the output P₁₁ of the NAND gate A₁, and which in turn quickly charges up the capacitor C₅ to provide a logic high capacitor voltage. Accordingly, the heating element HT₁ will be switched off.

After the heating element HT₁ has been switched off, either automatically or manually, the capacitor C₄ will discharge via the discharging path c-d. It is apparent that the heating element HT₁ can be switched again by momentarily closing the switch SW₂.

The invention is described by way of example only, and various modifications may be made without departing from the scope of the invention. 

What is claimed is:
 1. A hair curling appliance including a power control circuit for supplying electrical power from a power source to a heating element of the hair curling appliance, the power control circuit comprising: an input for connection to said power source, an output for connection to said heating element, and supply means between the input and the output for supplying power at a first, relatively higher level for a predetermined start-up time interval, and at a second, relatively lower level after the predetermined start-up time interval has elapsed.
 2. A hair curling appliance including a power control circuit as claimed in claim 1, wherein said supply means supplies power continuously to the heating element during the start-up time interval and intermittently thereafter so as to provide a time-averaged lower level of power.
 3. A hair curling appliance including a power control circuit as claimed in claim 1, wherein said supply means comprises first timing means for determining the start-up time interval, and second timing means for providing a signal to control the supply of power to said heating element.
 4. A hair curling appliance including a power control circuit as claimed in claim 3, wherein the second timing means is inhibited by the first timing means from providing a signal enabling control of the supply of power by the second timing means during the start-up time interval.
 5. A hair curling appliance including a power control circuit as claimed in claim 1, wherein said supply means includes timing means through which said supply means is triggerable to cease supplying power to the heating element after a predetermined time interval.
 6. A hair curling appliance including a power control circuit as claimed in claim 1, wherein said supply means includes manually operable switch means through which said supply means is triggerable to cease supplying power to the heating element.
 7. A hair curling appliance including a power control circuit as claimed in claim 1, further comprising a silicon controlled rectifier which is connected in series with said heating element for switching power supplied thereto.
 8. A hair curling appliance including a power control circuit as claimed in claim 1, further comprising rectifier means for converting an a.c. voltage from said power source to a d.c. voltage for said heating element.
 9. A hair curling appliance including a power control circuit as claimed in claim 2, wherein said supply means comprises first timing means for determining the start-up time interval, and second timing means for providing a signal to control the supply of power to said heating element.
 10. A hair curling appliance including a power control circuit as claimed in claim 9, wherein the second timing means is inhibited by the first timing means from providing a signal enabling control of the supply of power by the second timing means during the start-up time interval. 